Electrochemical fuel cells of the above-mentioned type convert reactants, namely a stream of hydrogen and a stream of oxygen, into electric power and water. Proton exchange membrane fuel cells (PEMFC) generally comprise a solid polymer electrolyte membrane disposed between two porous electrically conductive electrode layers so as to form a membrane electrode assembly (MEA). In order to induce the desired electrochemical reaction, the anode electrode and the cathode electrode each comprise one or more catalyst. These catalysts are typically disposed at the membrane/electrode layer interface.
At the anode, the hydrogen moves through the porous electrode layer and is oxidized by the catalyst to produce protons and electrons. The protons migrate through the solid polymer electrolyte towards the cathode. The oxygen, for its part, moves through the porous cathode and reacts with the protons coming through the membrane at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
FIG. 1 illustrates, in exploded view, a prior art proton exchange membrane fuel cell stack 10. Stack 10 includes a pair of end plate assemblies 15, 20 and a plurality of fuel cell assemblies 25. In this particular example, electrically insulating tie rods 30 extend between end plate assemblies 15, 20 to retain and secure stack assembly 10 in its assembled state with fastening nuts 32. Springs 34 threaded on tie rods 30 interposed between fastening nuts 32 and end plate 20 apply resilient compressive force to stack 10 in the longitudinal direction. Reactant and coolant fluid streams are supplied to, and exhausted from, internal manifolds and passages in stack 10 via inlet and outlet ports (not shown) in end plate 15.
Each fuel cell assembly 25 includes an anode flow field plate 35, a cathode flow field plate 40 and an MEA 45 interposed between plates 35 and 40. Anode and cathode flow field plates 35 and 40 are made out of an electrically conductive material and act as current collectors. As the anode flow field plate of one cell sits back to back with the cathode flow field plate of the neighboring cell, electric current can flow from one cell to the other and thus trough the entire stack 10. Other prior art fuel cell stacks are known in which individual cells are separated by a single bipolar flow field plate instead of by separate anode and cathode flow field plates.
Flow field plates 35 and 40 further provide a fluid barrier between adjacent fuel cell assemblies so as to keep reactant fluid supplied to the anode of one cell from contaminating reactant fluid supplied to the cathode of another cell. At the interface between MEA 45 and plates 35 and 40, fluid flow fields 50 direct the reactant fluids to the electrodes. Fluid flow field 50 typically comprises a plurality of fluid flow channels formed in the major surfaces of plates 35 and 40 facing MEA 45. One purpose of fluid flow field 50 is to distribute the reactant fluid to the entire surface of the respective electrodes, namely the anode on the hydrogen side and the cathode on the oxygen side.
One known problem with PEMFCs is the progressive degradation of performance over time. Actually, long-term operation of solid polymer fuel cells has been proven, but only under relatively ideal conditions. In contrast, when the fuel cell has to operate in a wide range of conditions, as is the case for automotive applications in particular, the ever-changing conditions (often modeled as load cycling and start-stop cycles), have been shown to reduce durability and lifespan drastically.
Different types of non-ideal conditions have been identified in the literature. A first of these conditions is referred to as “high cell voltage”; it is known that exposing a fuel cell to low or zero current conditions, leads to higher degradation rates in comparison to operation at an average constant current. A second non-ideal condition is “low cell voltage”; it is further known that drawing a peak current from the fuel cell also leads to increased degradation rates. It follows from the above that, in order to preserve the lifespan of a fuel cell, it is preferable to avoid both “high cell voltage” and “low cell voltage” operating conditions. In other words, the fuel cell should be operated only in a limited voltage range.
In order the cope with the abrupt changes in load that are typical of automotive applications, an electrochemical energy storage unit, such as a battery or a super capacitor, is usually associated with the fuel cell. The battery can work as a buffer: supplying electric power when there is a peak in the load and, conversely, storing excess electric power in case of low or zero load conditions. In principle, such an arrangement allows operating the fuel cell in the desired limited voltage range. However, once the battery is completely charged, it obviously ceases to be available for storing the excess electric power supplied by the fuel cell. A known solution to this last problem is simply to shut down the fuel cell until the level of charge of the battery reaches a lower threshold. However, start-stop cycles also contribute to the degradation of performance over time.